English is a language spoken by hundreds of millions of people and expecting them all to learn one monolithic language that is identical completely ignores the fact that people shape their language to suit their circumstances.

Most speakers speak several registers of their language in different levels of formality. I have a northern English regional dialect which has some features considered non-grammatical in "standard" English (for example people will say "I done" rather than "I did", or "Have you went...?" rather than "Have you been...?", or irregular forms of verbs such as "he telt" rather than "he told").

We even have a similar "incorrect conjugation" feature - people often use the third person instead of the first person when they are speaking narratively ("So I says to him... So I goes down the pub").

That doesn't mean I'd write those things in a formal setting, but it doesn't make them any less a form of the English language than any other.

You don't have to learn every dialect. Native English speakers don't learn every dialect. But given that most dialects operate within the same framework as other English dialects, if you know one you can guess the meaning from the context. If someone on American TV says "he have it" rather than "he has it" am I going to be sitting here wondering what they mean? No, of course not, because I know what they mean from the context and so the fundamental purpose of communication has been completed.